The present invention relates to a method for measuring and estimating a brake factor in a vehicle brake system, said vehicle comprising a towing vehicle and a trailer having a plurality of wheel axles, wherein said method comprises: initiating a controlled braking manoeuvre involving at least a first wheel axle and a second wheel axle; and obtaining values representing said brake factor for said first wheel axle and said second wheel axle by means of a control unit which is provided with a brake adaptation function to obtain a brake balance between said towing vehicle and said trailer.
The invention also relates to an arrangement for measuring and estimating a brake factor in a vehicle brake system, said vehicle comprising a towing vehicle and a trailer having a plurality of wheel axles, wherein said arrangement furthermore comprises a control unit adapted for initiating a controlled braking manoeuvre involving at least a first wheel axle and a second wheel axle, and for obtaining values representing said brake factor for said first wheel axle and said second wheel axle, said control unit being provided with a brake adaptation function to obtain a brake balance between said towing vehicle and said trailer.
Heavy motor vehicles, such as load-carrying commercial vehicles, are normally designed with different brake systems, for example wheel brakes (i.e. disc brakes), a hydraulic retarder and an engine brake.
Also, today's commercial vehicles are often arranged as vehicle combinations comprising a tractor, i.e. a towing vehicle, and a trailer. Such vehicles are often provided with an electronically controlled pneumatic brake system in order to control the wheel brakes. A brake system of this kind is normally based on a computerized control unit which may be arranged so as to distribute the requisite brake pressure in a suitable manner between the brakes of the tractor and the trailer. This distribution function is sometimes referred to as a “coupling force control”, or a “brake adaptation function”. By means of such a function, a high degree of brake compability, or brake balance, between the tractor and the trailer can be obtained. In other words, the aim with a brake adaptation function is to distribute the brake pressure in an optimum manner between the tractor and the trailer.
In the event of a poor brake balance, the stability, efficiency and safety of the vehicle combination will be influenced in a negative manner. As an example of an inferior brake balance, a situation can be mentioned in which the trailer does not brake sufficiently well, whereas the braking operation of the tractor is sufficient. As a result of this poor brake balance, an unwanted pushing force will arise in the coupling between tractor and trailer during braking. The tractor then has to brake both its own load and also a certain force from the trailer in order to achieve the necessary brake retardation. This means that the brakes of the tractor may become too warm, which causes them to be worn out too quickly. This means that the efficiency and stability of the brakes may be deteriorated.
On the other hand, if perfect brake balance is obtained, each axle is braking exactly its load and there is a high degree of compatibility between the tractor and the trailer.
In order to control the brake balance of the tractor and the trailer, the so-called “brake factor”, or “brake gain”, is used as an input parameter to the brake control unit. The brake factor, normally designated Bf, can be defined as a relation between the received brake torque and the applied brake cylinder pressure for a given wheel axle, i.e.Bf=Tbrake/Pcyl [Nm/bar/axle]wherein Tbrake indicates the received brake torque for the axle in question, and wherein Pcyl indicates the applied brake cylinder pressure for said axle. By determining a value which represents the brake factor Bf for each axle of a vehicle, the control unit may be operated so as to achieve the above-coupling force control. More precisely, tests can be carried out for a certain wheel axle wherein the applied brake pressure Pcyl (i.e. the pressure acting on a wheel brake disc by means of a corresponding brake pad) is measured while the retardation of the free rolling axle is also measured during braking. A value representing the retardation can be obtained by means of an accelerometer. By measuring the retardation, a value representing the brake torque Tbrake can be calculated. When calculating the brake torque Tbrake, certain factors such as the air resistance and the rolling resistance must be compensated for. By using the values of the brake torque Tbrake and the brake pressure Pcyl, the brake factor Bf can be calculated using the above-mentioned relationship.
Consequently, the brake factor Bf can be regarded as a value which represents the efficiency of the brakes. Also, a low brake factor may for example indicate possible malfunctions in the brakes. For example, the contact area of the brakes might be contaminated with dirt or rust, which means that it will need conditioning.
Generally it can be said that the purpose of an ideal braking system is to brake the vehicle in a controlled, stable and predefined way in order to stop the vehicle in the shortest possible distance no matter what the road, load or weather conditions are. Also, the various parts in the system should not become worn or become unnecessarily warm. In order to achieve this purpose, it is of great importance to obtain status information regarding the brakes. Such information can be obtained by means of the above-mentioned brake factor Bf.
Furthermore, it is previously known that the brake pressure distribution between the tractor and the trailer is based on an assumption that the brake factor Bf for each axle is known and also that it is considered to be constant This means that a brake control unit is provided with a constant value representing an approximation of the brake factor Bf for each wheel axle.
However, even though a constant value may be a relatively accurate approximation of the brake factor Bf for a given axle, it can be noted that relatively large variations of the brake factor Bf may occur during certain operating conditions of a vehicle. Variations in the brake factor Bf can easily lead to an inferior brake balance between the tractor and the trailer, which obviously is a disadvantage.
Consequently, there is a need for systems and methods for improving the above-mentioned coupling force control by using a correct input value representing the brake factor Bf during different operating conditions. In this manner, the brake pressure between the tractor and the trailer can be distributed in an improved manner.
According to prior art, it can be noted that the patent document U.S. Pat. No. 5,414,466 teaches a system for determining relative brake factors in a vehicle. The system is arranged so as to provide a value of the brake factor of a vehicle by means of a method in which the vehicle is decelerated while the braking pressure is measured and recorded for each braking location. However, a disadvantage with the system according to U.S. Pat. No. 5,414,466 is that it does not take into account the fact that the brake factor may vary in accordance with different operating conditions.
It is desirable to provide a method and an arrangement for measuring and estimating a brake factor in a vehicle brake system, which overcomes the above-mentioned drawbacks and problems associated with previously known devices.
A method according to an aspect of the present invention comprises the following steps: braking, in a forced manner, said first wheel axle when retardation of said vehicle is requested; estimating the brake factor for said first wheel axle by determining the brake pressure and braking torque for said first wheel axle; providing a transition phase after which said second wheel axle is braked, in a forced manner, and estimating the brake factor for said second wheel axle by determining the brake pressure and braking torque for said second wheel axle.
In an arrangement according to an aspect of the present invention, said control unit furthermore is adapted for carrying out a braking, in a forced manner, of said first wheel axle when retardation of said vehicle is requested, for estimating the brake factor for said first wheel axle by determining the brake pressure and braking torque for said first wheel axle, for providing a transition phase after which said second wheel axle is braked, in a forced manner, and for estimating the brake factor for said second wheel axle by determining the brake pressure and braking torque for said second wheel axle.
By means of the invention, certain advantages can be obtained. Firstly, it can be noted that the invention relies on the insight that the brake factor in fact varies depending on various factors, in particular the velocity, the load on the brake pad and the brake pad temperature. This means that by using this knowledge, the invention can be arranged for an improved brake adaptation function. In this manner, an improved brake compatibility between a towing vehicle and a trailer can be obtained. This may also lead to improved safety as well as a reduced repair cost due to bad distribution between truck and trailer.